Why FET amplifiers?

[Nikolas Ojala]

Nelson Pass, why do you favor field effect transistors over bipolars in your audio amplifiers? Is there some special reason that lead you to use MOSFET and JFET in your amplifiers?

 

[bear]

I'm not Nelson Pass, but my response is that JFets have a few advantages over bipolars:

- self biased with nothing on their gates
- triode like curves
- different spectra of distortion from bipolars
- different clipping characteristic than bipolars
- typically low noise

...others will likely suggest other factors as well

oh - they "sound good".

_-_-

 

[Zen Mod]

sole reason - jfets are generally more expensive than bipolar counterparts , so Papa can write bigger number on price tag .

 

[flg]

He also has produced many single or 2 stage designs and although bipolars have enough gain compared to FETs, they also require base current effectively lowering input impeadances very often requiring an extra stage. Simplicity is usually better.
In the output stage, it is my experience that MOSFETs have more reliability and rugedness without the second breakdown, thermal runaway etc.
Oh, I'm not Nelson either.

 

[Nelson Pass]

I used bipolars for the first 20 years of my work, but the
FET parts kept getting better, and ultimately they were a
better fit for the types of amplifiers I like to make.

If I didn't have them, I would work with bipolars.

 

[AndrewT]

In the output stage, it is my experience that MOSFETs have more reliability and ruggedness without the second breakdown, thermal runaway etc.

second breakdown under another name often occurs in MosFETs. Thermal runaway in vertical MosFETs afflicts them just as much as in Power BJTs.

 

[Nikolas Ojala]

Thanks

I used bipolars for the first 20 years of my work, but the
FET parts kept getting better, and ultimately they were a
better fit for the types of amplifiers I like to make.

If I didn't have them, I would work with bipolars.

Thanks.

The other answers were interesting too.

 

[CBS240]

second breakdown under another name often occurs in MosFETs. Thermal runaway in vertical MosFETs afflicts them just as much as in Power BJTs.

'Second breakdown' is not quite as limiting in mosfets wrt thier SOA. Thermal runaway is not quite as intense because of the limited transconductance of the mosfet compared to BJT, but still results in the same outcome.

 

[tmblack]

I understand germanium transistors have less Vbe =0.3V, but do germanium transistors have smoother turn on as well?
There are lots of Russian Ge transistors that some say have very little leakage.
Perhaps, if they are more linear than Si types then there could be a comeback for them.

Tom

 

[bear]

second breakdown under another name often occurs in MosFETs. Thermal runaway in vertical MosFETs afflicts them just as much as in Power BJTs.

Ummm... no.

Many Mosfets have a negative tempco until some threshold is reached beyond which they no longer can "turn off" on their own, and only then does thermal runaway occur.

Afaik, there are NO bipolars that exhibit this characteristic. Name one if there are Andrew.

All power circuits that use bipolars that produce substantial heat - like a power amp - include external devices to reduce the bias to prevent runaway. The typical Mosfet output stage does not.

_-_-bear

 

[iko]

I know this is mosfet 101and a bit off topic, but still, this app note is a nice refresher on mosfets and thermal instability.

http://www.onsemi.com/pub_link/Collateral/AND8199-D.PDF

Figure 4 is a nice view of the threshold, before it and after.